The market for miniaturized cameras used in smartphones and cellphones, tablets, portable computers, cars and trucks is rapidly expanding. Image quality requirements for state-of-the-art cameras force manufacturers to develop complex optical assemblies composed of many aspherical molded lenses.
FIG. 1 shows an exemplary optical assembly composed of four molded plastic lenses. In particular, this assembly, which is described in U.S. Pat. No. 7,777,972, includes four lenses arranged to form an image on a sensor located at an image plane 170 of the assembly. The lens elements are arranged in a first lens group composed of a first lens element 100 with positive refractive power having an aspheric convex object-side surface 101 and an aspheric convex image-side surface 102. The assembly also includes a second lens group composed of a second lens element 110, a third lens element 120, and a fourth lens element 130.
The second lens element 110 has negative refractive power having an aspheric convex object-side surface 111 and an aspheric concave image-side surface 112. The third lens element 120 has an aspheric concave object-side surface 121 and an aspheric convex image-side surface 122. The fourth lens element 130 has an aspheric convex object-side surface 131 and an aspheric concave image-side surface 132. An aperture stop 140 is positioned between the first lens element 100 and the imaged object. An IR filter 150 is disposed between the image-side surface 132 of the fourth lens element 130 and the image plane 170, the IR filter 150 having no influence on the focal length of the imaging optical lens assembly. A sensor cover glass 160 is arranged between the IR filter 150 and the image plane 170, the sensor cover glass 160 also having no influence on the focal length of the imaging optical lens assembly.
In general, the curved lens surfaces are rotationally symmetrical about an axis, and each surface's axis nominally lies on a common axis—the optical axis—of the assembly. Common centration of the lens surface axes is important for the optical performance of the overall assembly. Also important is the curvature of each lens surface and the spacing between each lens surface—i.e., both the lens thickness and the spacing between adjacent lenses.
Accordingly, each individual lens typically includes multiple centration and spacing datums, manufactured with tight enough tolerances to provide proper alignment of the final lens assembly when stacked together in addition to the curved functional optical surfaces depicted in FIG. 1. These datums are commonly provided by additional non-optically active portions of each lens, which form a ring around the edge of the active lens portions. When assembled, the non-optical portions of the lenses stack together, aligning and spacing the lens portions relative to each other as required by the overall lens assembly design.
Because of tightening manufacturing tolerance budgets, conventional metrology equipment (e.g., contact probes and gages, tactile profilers, inspection microscopes) is in many cases no longer capable of achieving required measurement reproducibility or accuracy. Additionally, metrology equipment for measuring certain properties of the lenses, such as refractive index or birefringence, on the production floor, is not commercially available. Accordingly, a metrology gap exists today.